For 5 points and a grenade slot this upgrade adds a new weapon to the unit with one black die with Impact 1 per figure at range 1.
Impact Grenades are situationally very strong. If the unit that adds them normally only rolls one white die then Impact Grenade is a significant upgrade in damage output. Otherwise the main use of this weapon is to take down vehicles. Keep in mind that Impact 1 is cumulative per model, so five troopers throwing Impact Grenades roll 5 black dice and have Impact 5.
This is a very useful upgrade for units that want to get close to their target and either need to be able to deal with Armor or increase their damage output at short range (if they normally roll less than one black die). Impact Grenades are essentially an auto-include on Snowtroopers because their flame thrower wants to be very close already and adds blast, making Concussion Grenades redundant. It is also a very strong upgrade on Stormtroopers increasing their white die to a black die if they get within range 1. It is not necessary on rebel troopers as they already roll a black die (unless your list has no other significant way of dealing with armor) and it is not reccomended on Fleet Troopers as their 2 white dice is on average more damage than 1 black die and have a decent chance of rolling a critical if necissary. Because grenades add a new weapon for 5 points, the more figures in the unit that can use the weapon, the more efficient the upgrade is.
Rebel Trooper Expansion, Fleet Trooper Expansion, Stormtrooper Expansion, Snowtrooper Expansion